1. Field of the Invention
The present invention relates to an electrostatic latent image developer (also simply referred to as a xe2x80x9cdeveloperxe2x80x9d) which can be used for electrostatic systems such as an electro-photographic printing system, an electrostatic recording system and an electrostatic printing process employed in, for example, copying machines and laser printers.
2. Description of the Related Art
Recently, a two-component developing system has been widely applied to a color printing system. Such two-component developing system involves a developer which comprises the proper amounts of magnetic carrier particles and an electrically insulting non-magnetic toner. Such an electrically insulting non-magnetic toner generally contains a polyester resin as a binder since the polyester resin has an excellent affinity for pigments, as described in JP-A-7-114204.
However, an excess amount of such polyester resin-containing toners may cause the undesirable staining on the non-imaging region of a transfer paper after transferred to a fuser roller, resulting in so-called xe2x80x9coffsetxe2x80x9d phenomena.
Therefore, such toners generally included a releasing agent, which may mainly comprise a polyethylene wax, a mixture of a polyethylene wax and a polypropylene wax, and a Fischer-Tropsch wax which is made from a coal, in order to improve the releasing property of a toner and the better toner fixation to a printing medium at lower temperature.
Further, a polypropylene wax may disadvantageously reduce the fixing property of the toner at lower temperature since a polypropylene resin has a melting point of as high as 130xc2x0 C. or higher as disclosed in, for example, JP-A-8-231640. In short, a toner containing only a polypropylene wax can be hardly fixed to a printing media at lower temperature. Alternatively, the amount of polypropylene wax to be contained in a toner must be extremely limited when the toner contains a polypropylene resin in combination with a polyethylene wax.
Therefore, in JP-A-11-272009, JP-A-11-15197, JP-A-10-123753, JP-A-8-82952 and JP-A-5-313493, it was suggested to add either a polypropylene wax having a low molecular weight or a uniform molecular weight, or both polypropylene wax and polyethylene wax to a binder resin and colorant in order to prevent the reduction in the fixing property of a toner at lower temperature.
However, neither that the toner containing a polypropylene wax can be used in combination with a resin-coated magnetic carrier having a coating layer produced by polymerizing a polyethylene resin directly on the surface of the carrier (described below) nor that such combination can provide the unique advantages was mentioned in any of these applications.
Alternatively, JP-A-2000-10337 and JP-A-11-282194 disclosed the toners in which a polyester wax is contained in both a binder resin and a colorant.
However, the toner disclosed in JP-A-2000-10337 had a disadvantage that some particular ester waxes having iodine number of 25 or less and saponification number of from 30 to 300 must be employed since the polyester wax is susceptible to oxidization. Further, the toner disclosed in JP-A-11-282194 had a disadvantage that fatty acid metal salt must be added in the same amount as that of polyester wax added due to the poor disperse-ability of a polyester wax.
In short, neither JP-A-2000-10337 nor JP-A-11-282194 could overcome these disadvantages since these inventions did not take into account the type of the carrier to be used for the toner.
On the other hand, the magnetic resin carriers which comprises the magnetic granule dispersed in a binder resin have been employed as a carrier in a two-component developing system in order to prevent the toner filming on the surface of the carrier, to form a uniform surface on the carrier, to improve the moisture resistance, to prevent the damage of photoreceptor by the carrier, and to provide a charge controlling ability, as disclosed in JP-A-1-282564 and JP-A-2-69770.
Such magnetic resin carriers, however, disadvantageously had a poor durability since the binder resin tends to be separated from the carrier after long-time use due to the poor binding force between the magnetic granules and the binder resin.
Accordingly, the resin-coated magnetic carrier is suggested to improve the durability of the carrier in JP-A-9-204075. The resin-coated magnetic carrier comprises the magnetic granules as the carrier core and a coating layer of a high molecular weight polyethylene resin on the surface of the magnetic granules. The coating layer may be formed by polymerizing a ethylene monomer directly on the surface of the magnetic granules by utilizing the catalysts carried on the surface of the magnetic granules. Thus, the obtained polyethylene layer has an excellent strength and the proper resiliency so that the layer can absorb impact such as stir, thus providing the better durability.
However, a two-component developing system which involves a developer comprising a toner containing the above-described wax and a resin-coated magnetic carrier (hereinafter simply referred to as a xe2x80x9ccarrierxe2x80x9d) had the higher possibility of the toner spent. In short, since a polyethylene resin or the like is contained in both of the toner and the carrier, the toner has an enhanced affinity for a carrier and thus tends to adhere to the carrier through the polyethylene wax or the like. Further, such resin-coated magnetic carrier had a disadvantage that it has a polyethylene coating layer formed on all the surface of the carrier core granules so that the polyethylene wax-containing toner is more adhered to the carrier.
Such toner spent will cause the decreasing of the charging ability, which possibly result in fogging and the decreasing of the image density. Therefore, such developer cannot have a prolonged life-time even if it comprises a carrier with a high durability.
The present invention was developed for solving the above-described problems. Thus, the present invention provides an electrostatic latent image developer which comprises a resin-coated magnetic carrier and a toner, wherein: said resin-coated magnetic carrier comprises a magnetic carrier core and a coating layer produced by polymerizing a polyethylene resin directly on the surface of the carrier core (a direct polymerization); and said toner contains either or both of a polypropylene wax and a polyester wax as a main wax component without substantially using a polyethylene wax.
Accordingly, the object can be achieved by the present electrostatic latent image developer that enables to prevent the toner spent as well as to maintain the excellent image density and T/C (toner/carrier) property over a long period of time even when the resin-coated magnetic carrier having a polyethylene coating layer is used.
Although it has conventionally been said that the toner containing a polypropylene wax had reduced the fixing property at lower temperature, it has been shown that this problem can be resolved by additionally using another particular type of the resin-coated magnetic carrier and adjusting the molecular weight of the resin or the like.
Also, though it has traditionally been said that charge control of toner containing polyester wax was difficult since polyester tends to be oxidized easily, it has been shown that this problem can be resolved by additionally using another particular type of the resin-coated magnetic carrier or by using a polypropylene wax in combination.
Other futures of the present inventions are described and recognized easily from the descriptions in the sub-claims.